1. Field of the Invention
The present invention relates to a method for manufacturing a capacitor embedded in a printed circuit board (PCB), and more particularly, to a method for manufacturing a capacitor embedded in a PCB, which can remove a surface defect of a copper clad lamination (CCL) substrate acting as a bottom electrode, thereby improving the yield of the capacitor.
2. Description of the Related Art
Discrete chip resistors or discrete chip capacitors have been mounted on a surface of a PCB. Recently, PCBs with embedded passive elements such as resistors or capacitors have been developed.
In such a PCB technique, passive elements such as resistors or capacitors are embedded in outer or inner layers of the PCB by using new materials and processes, and the embedded passive elements act as an existing chip resistor or chip capacitor.
For example, when a capacitor is buried in an inner or outer layer of the PCB and integrated as a part of the PCB regardless of a PCB size, the capacitor is referred to as an embedded capacitor and the PCB is referred to as an embedded capacitor PCB.
The most important characteristic of the embedded capacitor PCB is that the capacitor need not be mounted on a surface of the PCB because the capacitor is formed as a part of the PCB.
Three methods for the embedded capacitor PCB will be described below.
A first method is to manufacture a polymer thick film type capacitor by depositing, thermally hardening and drying a polymer capacitor paste.
According to the first method, a polymer capacitor phase is deposited on an inner layer of a PCB and is dried. Then, a copper paste is printed and dried to form an electrode. In this way, an embedded capacitor is manufactured.
A second method is to manufacture an embedded discrete type capacitor by coating a ceramic filled photo-dielectric resin on a PCB.
According to the second method, after photo-dielectric resin containing ceramic powder is coated on a substrate, a copper foil is laminated on the photo-dielectric resin to form a top electrode and a bottom electrode. Then, a circuit pattern is formed and the photo-dielectric resin is etched to form an embedded discrete type capacitor.
A third method is to manufacture a capacitor by inserting a separate dielectric layer having a capacitance characteristic into an inner layer of a PCB such that the dielectric layer can replace a decoupling capacitor which has been mounted on a surface of the PCB.
According to the third method, a power distributed decoupling capacitor is manufactured by inserting a dielectric layer with a power electrode and a ground electrode into the inner layer of the PCB.
Meanwhile, compared with an external capacitor, the capacitor embedded in the PCB is difficult to secure a sufficient capacitance because its size is limited according to a volume of the PCB.
Therefore, there is a demand for a technique for embedding a high-density capacitor in the PCB by implementing a high capacitance density per unit area. An example of the high-density capacitor is an external multi layered ceramic capacitor (MLCC) that is not embedded but mounted on the PCB. To this end, a thin film technology has been applied to the method for manufacturing an embedded capacitor in order to increase a permittivity of the dielectric layer but decrease a thickness thereof.
However, when the thin film technology is used to form the dielectric layer thinly about several hundreds of nanometers in order to minimize a size of the capacitor, a formation defect of the dielectric layer may occur according to a surface state of the bottom electrode disposed under the dielectric layer. This leads to an increase of a leakage current and an electrical short between the bottom electrode and the top electrode.
Hereinafter, problems of a conventional capacitor embedded in a PCB will be described below in detail with reference to FIGS. 1 and 2.
FIG. 1 is a cross-sectional view of a conventional capacitor embedded in a PCB. Referring to FIG. 1, the conventional capacitor 100 includes a CCL substrate 110, a dielectric layer 120, and a top electrode 130. The CCL substrate 110 includes a reinforcement member 111 (e.g., FR-4) and copper foils 112 formed on both surfaces of the reinforcement member 111. The CCL substrate 110 acts as a bottom electrode of the embedded capacitor.
A surface of the copper foil 112, that is, a surface of the CCL substrate 110 on which the dielectric layer 120 is formed, has surface defects such as a convex defect and a concave defect, depending on a surface state of the reinforcement member 111.
These surface defects increase a leakage current of the embedded capacitor, degrading the characteristic and reliability of an embedded capacitor PCB.
In addition, when the dielectric layer 120 is formed on the CCL substrate 110 having the surface defects, especially when it is formed thinly about several hundreds of nanometers in order to minimize a size of the capacitor, a defect occurs in the dielectric layer 120, as indicated by a reference symbol “F”. The defects will be described in detail with reference to FIG. 2.
FIG. 2 is a photograph illustrating the problem of the conventional capacitor embedded in the PCB. Specifically, FIG. 2 illustrates a convex defect of the CCL substrate in which a convex portion of the CCL substrate 110 passes through the dielectric layer 120 and the top electrode 130 and thus is exposed. In addition, an enlarged convex portion is illustrated in FIG. 2.
When the dielectric layer 120 and the top electrode 130 are sequentially formed on the CCL substrate 110 having the convex defect, the dielectric layer 120 cannot be formed in the convex defect. Therefore, the CCL substrate 110 acting as the bottom electrode is shorted to the top electrode 130.
That is, the surface defect of the CCL substrate increases the leakage current of the embedded capacitor and shorts the bottom electrode to the bottom electrode. Consequently, the characteristic and reliability of the capacitor embedded in the PCB are degraded and its manufacturing yield is reduced.